This invention relates to non-agglomerating, readily separable mixed bed ion exchange resin systems. More particularly it concerns a method for neutralizing both the surface charges of the anion exchanging component and the cation exchanging component of the mixed bed system by treatment with an anionic polyelectrolyte and a cationic polyelectrolyte.
The use of mixed bed ion exchange resins for deionization of aqueous solutions is widespread. Effective regeneration of mixed bed ion exchange resins is the key to their successful use, and the separation of the two components is the critical step. Surface interactions between the cationic and anionic resin particles cause clumps or agglomerates of resin to form that lead to poor flow distribution in the bed and hence inefficient operation. Further, during the backwash operation, when it is desirable to separate the cationic and anionic resins into layers, said separation is poor. The art has discovered ways to overcome this undesirable agglomeration by treatment with water-insoluble crosslinked ion exchange emulsion particles (U.S. Pat. No. 4,347,328), and treatment with water-soluble resinous polyelectrolytes to neutralize the surface charges of the treated resins (U.S. Pat. No. 2,961,417 and U.S. Pat. No. 3,168,486). Treatment with insoluble ion exchange emulsion particles (U.S. Pat. No. 4,347,328) involves high usage levels and extensive washing of the treated resin to remove the emulsion particles. Further, mixed bed resins prepared using Tasaki's approach (U.S. Pat. No. 5,902,833 and U.S. Pat. No. 6,060,526) are essentially non-agglomerating without affecting the kinetics of the anion exchange resin component.
The art has demonstrated lack of agglomeration in the sense that the volume of the mixed bed resin is comparable to the sum of the volumes of the individual components. However, lack of agglomeration does not guarantee good separation of anion and cation resins when these mixed bed resins are backwashed prior to separation and regeneration. A type of weak agglomeration is sometimes observed that does not affect the volume of the mixed bed resin. Specifically, loose agglomerates of anion and cation exchange beads are formed. These loose agglomerates are not broken apart during the backwash step and tend to reside in the cation resin layer when the bed is settled after backwashing. External visual observation does not show the presence of these agglomerates in the settled bed. Rather, the volume of the “cation” resin layer is substantially larger, and the volume of the anion resin layer is substantially less, than the volume of the corresponding resins originally charged. The presence of anion resin in the cation resin layer leads to extremely poor regeneration results and poor mixed bed performance on subsequent cycles. It is known to those skilled in the art that the presence of as little as 1% cross contamination can result in a severe deterioration of performance of said mixed bed on subsequent cycles such that the purity of the water produced is unacceptable. For example, water purity is especially important in the semiconductor industry where the extent of miniaturization depends on ultra pure water.
The present invention overcomes the problems of the prior art by providing a composition for and process for preparing non-agglomerating, readily separable mixed bed ion exchange systems.